The present invention relates to the field of radiocommunication with mobile stations. It is applied in particular in mobile station locating systems.
The mobile station locating function of a radiocommunication system consists in estimating the geographical position of a given mobile station, in particular on the basis of the signals which it exchanges over the air interface with a radiocommunication infrastructure. This function has undergone recent developments to take account of the need to locate with accuracy a mobile station whose user is making an emergency call. Thus, there are different locating strategies, some of which (the “cell ID” method, OTDOA (“Observed Time Difference Of Arrival”), location method using the GPS system (“Global Positioning System”)) are described in particular in the technical specification 3G TS 25.305, “Stage 2 functional specification of UE positioning in UTRAN”, version 3.8.0 published in March 2002 by the 3GPP (“3rd Generation Partnership Project”), and in the reference work “Principes de radiocommunication de troisième génération” [“Third-generation radiocommunication principles”] by M. Lucidarme, ed. Vuibert, 2002.
FIG. 1 illustrates the access network architecture envisaged for the location function in the context of GSM/GPRS networks for a GERAN-type network, a general description of which is provided in technical specification 3GPP TS 43.051, “GSM/EDGE Radio Access Network (GERAN), Overall Description—Stage 2 (Release 4)”, version 4.0.0, published in November 2000 by the 3GPP.
The GERAN network shown in FIG. 1 is built on a GSM infrastructure, and is conventionally divided into a core network (20) and a radio access network, also referred to as the BSS (“Base Station Subsystem”).
A general description of the radio interface, referred to as Um, between the mobile stations (MS) 23 and the base stations (BTS) 22 of the BSS is provided in technical specification ETSI TS 101 350, “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Overall description of the GPRS radio interface; Stage 2 (GSM 03.64, version 8.5.0, Release 1999), published by ETSI (European Telecommunications Standards Institute) in August 2000.
Each base station 22 is supervised by a base station controller (BSC) 21 via an interface known as Abis.
Different location methods are standardized for the GERAN network: the Timing Advance method, the E-OTD (“Enhanced Observed Time Difference”) positioning mechanism, the location method using the GPS system, and the U-TDOA (“Uplink Time Difference of Arrival”) method. FIG. 1 shows the functional entities of the location service which allow these different methods to be carried out. The GERAN network thus comprises location measurement units (hereafter referred to as LMU) 25a, 25b, which are respectively associated with BSCs, for which they carry out location-related measurements and processing. The figure shows two remote LMU-As 25a, 25c which operate autonomously and communicate with their associated BSC via an interface referred to as Um, and the Abis interface, while the LMU-B 25b, integrated in a base station 22, communicates with its associated BSC via the Abis interface. An entity 26 referred to as the SMLC (“Serving Mobile Location Centre”) is also provided to manage the location function. The placing of the SMLC function within the BSS is not standardized, so that an SMLC may be an autonomous network element of the GERAN network. In the example shown in FIG. 1, the two options are shown (SMLC 26 integrated into a BSC 21, and an autonomous SMLC 26 which communicates with the BSS via an interface referred to as Lb). Location methods and the location service architecture for the GERAN network are described in technical specification 3GPP TS 43.059, “Technical Specification Group GSM/EDGE Radio Access Network; Functional stage 2 description of Location Services (LCS) in GERAN (Release 6)”, version 6.0.0, published by the 3GPP in April 2003.
The aforementioned different location strategies do not offer the same performance, and meet different needs. The timing advance or cell ID methods are based on the determination of the serving base station of a given mobile station whose geographical coverage provides a first approximation of the station location. This method, which is advantageous by virtue of its simplicity, obviously lacks precision for certain applications. The GPS method can only be used with mobile stations which are equipped with receivers capable of receiving GPS signals. Its practical implementation further requires the supply of specific data, known as GPS assistance data, by the network infrastructure to the mobile station, in order to improve significantly the performance of the GPRS receiver installed in the mobile station.
The TOA (“Time of Arrival”) or TDOA (“Time Difference of Arrival”) location methods entail a measurement of the arrival time of the received signals. The presence of multipath propagation limits the accuracy with which the time of arrival of the first received signal component can be estimated. This has a significant impact on the performance of the entities responsible for calculating the location of the mobile stations in the network, and imposes a compromise between service accuracy and processing time.
The known methods for estimating the time of arrival of signals are based on the training sequence conventionally comprising bits known a priori to the receiver.
An object of the present invention is to propose methods for estimating the time of arrival of signals which offer improved performance while being particularly suitable for implementation in the context of the location of the mobile stations of a radiocommunication network.